Solvent recovery in solvent extraction



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Carl C. Georgian ATTORNE United States Patent SOLVENT REcovEnY 1N SOLVENT EXTRACTION Carl C. Georgian, La Marque, Tex., assignor, by mesne assignments, to The American Oil Company, a corporation of Texas Application June 8, 1953, Serial No. 360,397

2 Claims. (Cl. 196l4.17)

This invention relates to solvent extraction of hydrocarbon mixtures. More particularly the invention relates to the recovery of the solvent from the extract phase formed in the solvent extraction of hydrocarbon mixtures.

Hydrocarbons naturally occur as mixtures of the various classes of hydrocarbons, namely, parafiinic, olefinic, naphthenic and aromatic. Usually the mixture contains slight amounts of compounds containing oxygen and/or sulfur. These mixtures have been separated into more or less pure classes of compounds by means of selective solvents such as phenol, liquid S02, glycols, liquid propane, alcohols, etc.,Generally the solvent is recovered from solution with the extract hydrocarbons by evaporation and/or distillation. The usefulness of certain of the well known solvents is seriously limited by relatively low decomposition temperatures.

Recently certain of the glycols have been used in commercial processes for the recovery of aromatic hydrocarbons from petroleum naphthas in spite of the relatively low decomposition temperature of these glycols. The incipient decomposition temperature of diethylene glycol is 327 F. and of triethylene glycol is 403 F. In this process operation at decomposition temperature is avoided by the use of a solution of glycol and water, which solution has a boiling point markedly lower than the boiling point of the glycol itself.

Another Well known method for avoiding operation at the decomposition temperature of glycols takes advantage of the relatively low miscibility of aromatic hydrocarbons and glycols at ordinary atmospheric temperatures. In this process a solution of polyethylene glycol and Water is used to extract aromatic hydrocarbons from petroleum naphtha at temperatures between about 200 and 300 F., the solvent-aromatic hydrocarbon phase is separated and the aromatic hydrocarbons forced out of solution in the solvent by cooling the extract phase to about 100 F. Obviously this method is uneconomic in regard to heat consumption.

An object of this invention is the separation of hydrocarbon mixtures by treatment thereof with a glycol solvent. Another object is the recovery of glycol solvent from a solution of glycol and hydrocarbons under conditions avoiding thermal decomposition of said solvent. A particular object is the recovery of glycol solvent for reuse in the extraction process without lowering the temperature of the solvent-hydrocarbon solution appreciably below the temperature of the solvent-feed contacting zone. A more particular object is the recovery of glycol solvent by a liquid-liquid extraction operation utilizing a liquid hydrocarbon oil to remove extract hydrocarbons from the glycol-hydrocarbon solution and thereby regenerate the glycol solvent. A preferred object is a combination process wherein a petroleum naphtha which contains aromatic hydrocarbons is extracted with a polyethylene glycol to obtain an extract phase, and the glycol is regenerated for reuse in the process by contacting the extract phase with a liquid hydrocarbon oil that is essentially immiscible in the glycol and is miscible with the extract hydrocarbons and recovering the extract hydrocarbons from solution in said oil by distillation. Other objects will become apparent in the course of the detailed description of the invention.

It has been found that glycol suitable for reuse in a solvent extraction step can be obtained from a glycol-hydrocarbon solution by contacting said solution with a liquid hydrocarbon oil which is essentially immiscible in said solvent and is miscible with the extract hydrocarbons in an amount sufiicient to remove essentially completely the extract hydrocarbons from the glycol-hydrocarbon solution. Preferably the liquid hydrocarbon oil has a boiling range markedly difierent from the boiling range of the extract hydrocarbons in order that the extract hydrocarbons may be readily separated therefrom by distillation.

The feed to the process of this invention consists of a mixture of hydrocarbons from at least two of the broad classes, paraffinic, naphthenic, olefinic and aromatic, such as naturally occur in petroleum or the products from bydrocarbon conversion processes. A preferred feed consists of petroleum naphthas containing large amounts of aromatic hydrocarbons, e. g., catalytically cracked naphthas and the naphtha boiling range material from catalytic reforming in the presence of hydrogen. Examples of these stocks are hydroformate and platformate. The process is also applicable to the separation of olefins and diolefins as in the separation of butadiene from admixture with butanes and butenes or to the separation of styrenes from ethylbenzene and xylenes. Other feeds which may be charged to this process are kerosenes, heater oils, gas oils and lube oil distillates. In general the process is ap plicable to any feed which is susceptible to separation by treatment with a glycol solvent.

The selective solvents utilized in this process are broadly known as glycols, i. e., the dihydric alcohols. Particularly desirable solvents are the polyglycols, i. e., polymeric glycols containing ether linkages. The halogen and the other derivatives of glycols are also suitable solvents for this process. Particular examples of the solvents are ethylene glycol, diethylene glycol, triethylene glycol, tetraethylene glycol, propylene glycol, dipropylene glycol, ethylene glycol monomethyl ether, ethylene glycol diethyl ether, propylene glycol monoethyl ether. The preferred solvents are the polyethylene glycols. v

The solvent power of these glycols improves at elevated temperatures. Therefore, normally a selective solvent extraction process using glycol solvent operates at temperatures between about F. and the incipient thermal decomposition temperature of the particular glycol. More usually the contacting Zone is maintained at a temperature between about 200 and 300 F. It is to be understood that certain of the polyglycols are solids at ordinary temperatures and the temperature maintained in the process must be at least high enough to maintain these particular glycols in the liquid state. Furthermore, the process is a liquid-liquid operation and sufiicient pressure must be maintained on the system to keep the solvent, the feed and the hydrocarbon oil in the liquid state.

It has been found that the glycol solvent can be recovered for reuse in the process by contacting the glycolhydrocarbon solution, i. e., extract phase, with a liquid hydrocarbon oil which is essentially immiscible with the glycol and is miscible, or substantially so, with the extract hydrocarbons. Sufiicient hydrocarbon oil should be used to remove essentially completely the extract hydrocarbons from the glycol. The use of smaller amounts of hydrocarbon oil results in circulation of extract hydrocarbons in the system and a decrease in overall yield of the desired extract hydrocarbons. Since the extract hydrocarbons are usually the desired product of the process, it is preferred that the liquid hydrocarbon oil be readily separable from these extract hydrocarbons; preferably the liquid hydrocarbon oil boils in a range appreciably different from the boiling range of the extract hydrocarhens-thereby the hydrocarbon oil and the extract hydrocarbons are readily separable by distillation.

The solubility of paraffinic hydrocarbons in glycols is extremely low. Naphthenic hydrocarbons are also only very slightly soluble in glycols. These hydrocarbons are essentially completely miscible with virtually all hydrocarbons for which glycols have appreciable solvent power. The most suitable liquid hydrocarbon oil for the glycol recovery step of this process consists substantially of parafiinic hydrocarbons, naphthenic hydrocarbons and mixtures thereof. The liquid hydrocarbon oil may have a boiling range either lower than or higher than the boiling range of the extract hydrocarbons. It is preferred to use a hydrocarbon oil having a higher boiling range.

Suitable hydrocarbon oils may be obtained either from special crud es or more usually by the refining of distillates from ordinary crudes. Examples of suitable hydroc'arbon oils are: The highly refined product obtained by acid treating or liquid S02 extraction of kerosene distillates. The white oils, i. e., either technical grade or white mineral grade oils, meeting the U. S. Pharmacopoeia standards obtained by very heavy acid treating of various distillates. Lubricating oils obtained by selective solvent extraction of distillates from paraflinic and naphthe'nic-type crudes. Paraflin waxes are particularly suitable when operating at temperatures appreciably above the melting point of the wax.

When the feed to the process consists of a mixture of olefins and diolefins the diolefins may be recovered from the extract phase by contacting the extract phase with a liquid hydrocarbon oil consisting either of olefins or paraflins or naphthenes. That is, a liquid hydrocarbon oil essentially immiscible in the glycol solvent under the conditions of contacting of the oil and the extract phase.

The preferred liquid hydrocarbon oil is parafiinic or na'phthenic or mixture thereof which boils appreciably above the boiling point of the extract hydrocarbons.

The annexed drawing which is made a part of this specification shows an illustrative embodiment of the process of this invention. It is to be understood that many items of equipment and details of operation are omitted where these can be readily supplied by those skilled in this art.

In the drawing the feed is a hydroformate naphtha obtained from the cracking of a virgin heavy naphtha over a cobalt molybdena catalyst in the presence of hydrogen. This hydroformate has an ASTM end point of about 385 F. and contains benzene, toluene, ethylbenzene and xylene as the aromatic hydrocarbon constituents. Substantially no oleiinic hydrocarbons are present in this feed.

Feed from source 11 is passed through line 12 through heat exchanger 13 wherein the temperature is raised by i ndirect contact with hot rafiinate phase. From exchanger 13 the feed is passed by way of line 14 through heat exchanger 16, line 17 and manifold 18a, 18b, 180 into extractor 19. In heat exchanger 16 the feed is raised to about the temperature of operation of extractor 19.

In this embodiment the feed is introduced at about the vertical mid-point of extractor 19 by way of valved line 18b. However, the feed may be introduced at various points along the height of extractor 19 as illustrated by lines 18a, 18b and 18c. The type of feed and operating conditions will determine the exact point of feed inlet into extractor 19.

Extractor 19 is a vertical tower equipped to provide intimate contact of two immiscible liquids of different densities. Extractor 19 may be filled with conventional tower packing such as Ras'chig rings, Berl saddles, spheres, or may be provided with contacting trays such as bubble trays or perforated plates.

Extractor 19 is' operated at an elevated temperature appreciably lower than the incipient decomposition temperature of the triethylene glycol solvent used herein. In this embodiment extractor 19 operated at a temperature of 275 F. Although extractor 19 may in some cases be operated with a temperature gradient over the height of the vessel, in this embodiment a substantially constant temperature is maintained throughout the vessel.

In some cases it is desirable to operate with a glycolwater solution as the selective solvent, e. g., a solution of diethylene glycol containing between about 5 and 10 volume percent of water. In this embodiment an essentially anhydrous triethylene glycol solvent is used.

The amount of solvent used per volume of feed is dependent upon the type of feed, the type of glycol and operating conditions in extractor 19. In general the amount of solvent used may be between about 1 and 25 volumes per volume of feed, preferably between about 5 and 15 volumes. In this embodiment 8 volumes of triethylene glycol are used per volume of feed.

In order to improve the selectivity of extraction a reflux stream of aromatic hydrocarbons is introduced near the bottom of extractor 19 by way of valved line 27, heat exchanger 28 and line 29. The aromatic hydrocarcarbons used as reflux have the same composition as the aromatic hydrocarbon product; The amount of reflux used will vary with the degree of selectivity desired on a particular piece of equipment. In general it will be between about 2 and 10 volumes per volume of aromatic hydrocarbon extracted fromthe feed, i. e., product aromatic hydrocarbon; herein the ratio of reflux to product is 6:1.

Although the selectivity of the glycol solvent is very good, some non-aromatic hydrocarbons are extracted from the feed. These non-aromatic hydrocarbons boil in the range of the aromatic hydrocarbons and cannot be separated by fractional distillation. In order to improve the purity of the aromatic hydrocarbon product, it is desirable to remove these close boiling non-aromatic hydrocarbons- This is accomplished in this embodiment by introducing a wash oil into extractor 19 just above the extract phase outlet. The wash oil should be a hydrocarbon that is easily separable from the product aromatic hydrocarbons by distillation. In this embodiment pentane is used as the wash oil. Pentane is introduced into extractor 19 by way of line 32, heat exchanger 33 and line 34. Makeup, or fresh, pentane may be introduced from source 36 into line 32 by way of valved line 37. In this embodiment the amount of pentane introduced into extractor 19 is about 1 volume per volume of aromatic hydrocarbon product. Of course this amount may be varied in accordance with the requirements of any particular operation.

A raffinate phase consisting of substantially dearomatized feed naphtha and some occluded and dissolved solvent is withdrawn from the top of extractor 19 by way of line 41. This hot rafiinate phase is passed through heat exchanger 13 where it raises the temperature of fresh feed, thereby saving heat. The cooled rafiinate phase is passed from heat exchanger 13 into line 42 where it is contacted with water from source 43 and line 44. The water-rafiinate phase stream is intimately agitated in mixer 46 for a time sutficient for the water to dissolve the solvent present in the rafiinate phase. The amount of water used must be at least enough to form a separate aqueous phase. In this illustration about 5 volume percent of water based on rafiinate phase is used. A countercurrent tower may be used to contact the water and raffinate phase and thereby lessen the water usage.

The contents of mixer 46 are passed by way of line 47 into separator 48. In separator 48 the aqueous phase of water and solvent separated from the raflinate hydrocarbons and' is withdrawn by way of line 51. Raffinate hydrocarbons are withdrawn from separator 48 by way of line 52 and are passed to storage not shown.

Under some conditions of operation an essentially glycol-free raflinate can be obtained by simple settling of the rafiinate phase at ordinary atmospheric temperatures. Water washing is used to recover the glycol solvent in this embodiment in order to minimize losses of solvent. The aqueous solution may be stripped of water by various methods well known to the art and then returned to the process. When operating with a glycol-water solvent the glycol-water solution may be returned directly to the process in order to provide makeup water to the system.

Extract phase is withdrawn from the bottom of extractor 19 by way of line 53 and is passed into an upper portion of extractor 54. Extractor 54 is a vessel similar to construction to vessel 19. Makeup liquid hydrocarbon oil from source 57 is passed by way of valved line 58 into valved line 59 where it meets a circulating stream of liquid hydrocarbon oil. The oil in line 59 is passed through heat exchanger 61 and through line 62 into a lower portion of extractor 54. In heat exchanger 61 the liquid hydrocarbon oil is dropped to the temperature of operation of extractor 54 which is about that of extractor 19, i. 12., about 275 F. Normally the temperature in extractor 54 will be a few degrees lower than that of extractor 19 because of unavoidable heat losses.

The liquid hydrocarbon oil used in this embodiment is a solvent-extracted heavy kerosene essentially free of aromatic hydrocarbons. This kerosene boils between about 450 and 575 F. The amount of kerosene used is dependent upon the type of aromatic hydrocarbons present in extractor 54, the type of solvent used and operating conditions in extractor 54. The amount of oil used may be between about 1 and 25 volumes of oil per volume of aromatic hydrocarbons present in the extract phase introduced into extractor 54. In this embodiment 9 volumes of kerosene are used per volume of aromatic hydrocarbons.

A second raffinate phase is withdrawn from extractor 54 by way of line 64. This second raflinate phase consists of regenerated triethylene glycol solvent and contains only a trace amount of hydrocarbon material. The regenerated solvent is recycled to extractor 19 by way of valved line 23, heat exchanger 24 and line 25, or the solvent may be withdrawn from the system by way of valved line 66.

A second extract phase is withdrawn from extractor 54 by Way of line 68. This second extract phase consists essentially of kerosene, aromatic hydrocarbons, pentane and trace amounts of solvent. The extract phase in line 63 is passed through heat exchanger 61 and line 69 into fractionator 71.

Fractionator 71 is provided with reboiler 72. Fractionator 71 shows schematically the distillative separation of the second extract phase into a pentane fraction, an aromatic hydrocarbon fraction and a bottoms fraction. A pentane fraction is taken overhead from fractionator 71 by way of line 74 and is recycled to extractor 19 by way of valved line 32, heat exchanger 33 and line 34, or pentane may be removed from the system by way of valved line 76.

In this embodiment all of the aromatic hydrocarbons present in the second extract phase are shown as being removed by way of a side-draw-ott from fractionator 71. This aromatic hydrocarbon fraction is withdrawn by way of line 78. The requisite amount needed for reflux in extractor 19 is withdrawn and is passed by way of line 27, heat exchanger 28 and line 29 to extractor 19. The product aromatic hydrocarbons are passed by way of line 79 to storage and further processing not shown.

The product aromatic hydrocarbons may be separated by distillation into a benzene fraction, a toluene fraction and a Ca aromatic hydrocarbon fraction. Nitration grade materials may be obtained either by acid treating each fraction or by vapor phase clay treating.

The bottoms fraction from fractionator 71 consists essentially of kerosene. This bottoms fraction is withdrawn by way of line 82 and is recycled to extractor 54 by way of valved line 59, heat exchanger 61 and line 62. In

heat exchanger 61 the hot recycle oil is reduced in temperature to about 275 F. by indirect contact with the second extract phase from line 68. The bottoms from fractionator 71 may be withdrawn from the system by way of valved line 84.

The following example is cited to illustrate the results obtainable by this invention:

An extract phase was prepared by dissolving benzene in triethylene glycol. The synthetic extract phase consisted of volume percent of triethylene glycol and 20 volume percent of benzene. Triethylene glycol was selected for this illustration because of its greater solvent power for benzene at the temperature to be used in the test.

The liquid hydrocarbon oil used in this test was an acidtreated kerosene. This kerosene is characterized below. The presence of about 9 volume percent of aromatic hydrocarbons is particularly pointed out:

ASTM distillation, F.:

The extract phase was contacted countercurrently with the oil in a York-Scheibel extraction tower which provided about 40 mechanical stages. The extract phase was charged near the top of the extractor at a rate of 200 ml. per minute. The oil was charged near the bottom of the extractor at a rate of 300 ml. per minute. Thus 7.5 volumes of oil were used per volume of benzene present in the extract phase. The extractor was maintained at a uniform temperature of 275 F.

The benzene-oil phase was withdrawn from the top of the tower and was distilled. The distillation column was 8 inches in diameter and contained 12 feet of one-half inch Berl saddles. A reflux ratio of 10:1 was used. Stripping steam was introduced into the column because the maximum attainable bottoms temperature was only 347 F. The oil completely freed of benzene was recovered as a bottoms product from the distillation operation. A benzene product containing no oil was recovered from the top of the column.

The glycol phase removed from the bottom of the extractor was carefully fractionated and found to contain only 0.6 volume percent of hydrocarbon material. Analysis of this hydrocarbon material indicated that about 20 volume percent consisted of aromatic hydrocarbons boiling above benzeneapparently derived from the aromatic hydrocarbon constituents of the kerosene. This regenerated triethylene glycol can be used as a selective solvent with essentially no impairment of extraction efficiency.

A separate test was made on the solvent power of a kerosene-benzene solution for triethylene glycol. A kerosenebenzene solution containing 40 volume percent of henzene was contacted with triethylene glycol at 275 F. It was found that the kerosene-benzene solution did not dissolve a measurable amount of the triethylene glycol.

Thus having described the invention, what is claimed is:

1. An aromatic hydrocarbon recovery process which comprises (1) contacting a liquid petroleum naphtha containing appreciable amounts of aromatic hydrocarbons with liquid triethylene glycol solvent under conditions to 7 form extract and rafiinate phases, (2) separating, an extract phase comprising solvent and aromatic hydrocarbons from a. rafiinate phase, (3) contacting said extract phase with. a kerosene essentially free of aromatic hydrocarbons under conditions to form a second rafiinate phase consisting essentially of triethylene glycol solvent and a second extract phase consisting essentially of aromatic hydrocarbons and kerosene, (4) separating said second raffinate phase from said second extract phase and recycling said second rafiinate phase to the naphtha contacting zone (1 (5 recovering aromatic hydrocarbons by distillation from said second extract phase and (6) recycling the recovered kerosene to the extract phase contacting zone (3), wherein said naphtha contacting zone and said extract phase contacting zone are operated at substantially the same temperature.

2 The process of claim 1 wherein the, volume ratio of kerosene to aromatic hydrocarbon present in said extract phase; charged to said extract phase. contacting zone (3) is about 8.

References Cited in the file of this patent UNITED STATES PATENTS 2,114,524 Egli Apr. 19, 1938 2,215,915 Cope et al Sept. 24, 1940 2,302,383 Stratford et a1 Nov. 17, 1942 2,379,334 Atwell June 26, 1945 2,656,301 Findlay Oct. 20, 1953 FOREIGN PATENTS 441,104 Great Britain Jan. 13 1936 

1. AN AROMATIC HYDROCARBON RECOVERY PROCESS WHICH COMPRISES (1) CONTACTING A LIQUID PETROLEUM NAPHTHA CONTAINING APPRECIABLE AMOUNTS OF AROMATIC HYDROCARBONS WITH LIQUID TRIETHYLENE GLYCOL SOLVENT UNDER CONDITIONS TO FORM EXTRACT AND RAFFINATE PHASES, (2) SEPARATING AN EXTRACT PHASE COMPRISING SOLVENT AND AROMATIC HYDROCARBONS FROM A RAFFINATE PHASE, (3) CONTACTING SAID EXTRACT PHASE WITH A KEROSENE ESSENTIALLY FREE OF AROMATIC HYDROCARBONS UNDER CONDITIONS TO FORM A SECOND RAFFINATE PHASE CONSISTING ESSENTIALLY OF TRIETHYLENE GLYCOL SOLVENT AND A SECOND EXTRACT PHASE CONSISTING ESSENTIALLY OF AROMATIC HYDROCARBONS AND KEROSENE, (4) SEPARATING SAID SECOND RAFFINATE PHASE FROM SAID SECOND EXTRACT PHASE AND RECYCLING SAID SECOND RAFFINATE PHASE TO THE NAPHTHA CONTACTING ZONE (1), (5) RECOVERING AROMATIC HYDROCARBONS BY DISTILLATION FROM SAID SECOND EXTRACT PHASE AND (6) RECYCLING THE RECOVERED KEROSENE TO THE EXTRACT PHASE CONTACTING ZONE (3), WHEREIN SAID NAPHTHA CONTACTING ZONE AND SAID EXTRACT PHASE CONTACTING ZONE ARE OPERATED AT SUBSTANTIALLY THE SAME TEMPERATURE. 